In recent years, portable terminals used for mobile communication have been increasingly reduced in size and lightened in weight. Along with this, the portable terminals are being made multi-banded to be able to handle a plurality of communication systems and are being given more functions. Therefore, the built-in circuits have increased. For this reason, it is strongly demanded that the electronic parts used in the terminals become surface mount parts or small-size parts for improvement of their mounting density.
SAW devices have been known as key parts of portable terminals. A SAW device usually has a configuration where a sealed hollow portion which becomes a vibration space is secured in the vicinity of an electrode surface at which the SAW is excited. For such SAW devices as well, surface mountable small-size parts are being demanded.
With respect to such demands, a surface mount type SAW device as shown in FIG. 9 is proposed. The SAW device shown in FIG. 9 has a protective cover 32 having a frame member 32a surrounding a SAW element 31 on a piezoelectric substrate 30, thereby secures a vibration space 33 of the SAW element 31.
Further, on the main surface of the piezoelectric substrate 30 in the vicinity of the frame member 32a of the protective cover 32, columnar electrodes 34 electrically connected to the SAW element 31 are provided. The columnar electrodes 34 are provided in order to guide electric signals of the SAW element 31 to an external circuit or introduce electric signals to the SAW element 31 from the outside. The columnar electrodes 34 are sealed by a sealing resin 35 molded around them. This sealing resin 35 prevents breakage of the columnar electrodes 34 when shock is applied from the outside when mounting or using the SAW device (see, for example, Patent Document 1, FIG. 22 to FIG. 24).
In the SAW device shown in FIG. 9, however, voids 36 such as air bubbles or the like easily form around the columnar electrodes 34 for the reason that the resin is not sufficiently filled in the clearance between the protective cover 32 and the columnar electrodes 34. Thus the support strength of the columnar electrodes 34 falls, and the columnar electrodes 34 break more easily at the time of mounting etc. of the SAW device. As a result, a drop in the reliability of the SAW device is invited because connection failure etc. easily occur when the columnar electrodes 34 break.
Further, in the case of the SAW device shown in FIG. 9, a contact area of the frame member 32a and the piezoelectric substrate 30 is small, and a bonding force of the protective cover 32 and the piezoelectric substrate 30 is relatively weak. Accordingly, when stress due to a difference of coefficients of thermal expansion between the sealing resin 35 and the protective cover 32 or the like is added to the protective cover 32, the protective cover 32 easily peels off from the piezoelectric substrate 30. As a result, a drop in the reliability of the SAW device is invited.
Above, a SAW device was exemplified as the acoustic wave device, but other acoustic wave devices, for example, thin film bulk acoustic resonators, also encounter problems similar to those described above.    Patent Document 1: Japanese Patent Publication (A) No. 2007-325013